Background and Goal of Study: Ultrasonography might be a tool to assess airway difficulty1. However, no validated measurement of submandibular space compliance as part of airway assessment has been reported2. We investigated whether two indices, derived from the ultrasound-based quantification of the distances between the hyoid bone and mandible (HMD) in neutral, sniffing, and maximal hyperextended positions, may be used to predict the occurrence of Cormack 3 and 4 grades during direct laryngoscopy.
Materials and Methods: After the Ethics Committee approval and signing the informed consent forms, fifty adult patients scheduled to undergo general anaesthesia with oro-tracheal intubation were prospectively included. Exclusion criteria were morbid obesity and rapid sequence inductions. We measured the HMDs in neutral, sniffing, and maximal hyperextended positions, in mid-sagital plane, using a curvilinear ultrasound transducer. We calculated the ratio between HMDs in sniffing and neutral postions (Compl1) and the ratio between the HMDs in maximal hyperextended and neutral positions (Compl2). All intubations were performed in sniffing position, with Macintosh curved laryngoscope blades, with no external laryngeal manipulation. We used ROC (receiver operating characteristics) curve analysis.
Results and Discussion: For Compl1, we identified an optimal threshold of 1.06, with 75% sensitivity, and 97.83 specificity. For Compl2, we identified a threshold of 1.24 as being most disciminative between patients with and without difficult intubation, with 100% sensitivity and 71.7% specificity. Thus, the lengthening of the submandibular space (HMD) by less than 24% from neutral to maximal hyperextended position had the highest sensitivity in predicting the occurrence of Cormack grades 3 and 4. The areas under the ROC curve were 0.87 for Compl1 and 0.91 for Compl2 (p=0.65).
Conclusion: Ultrasound-based quantification of submandibular space compliance might be a useful tool, with good sensitivity and specificity, in the prediction of difficult airway.
References:
1Fulkerson J, Moore H, Anderson T, Lowe R Jr. Ultrasonography in the preoperative difficult airway assessment. J Clin Monit Comput 2016; DOI 10.007/s10877-016-9888-7.
2Greenland KB. Airway assessment based on a three column model of direct laryngoscopy. Anaesth Intensive Care 2010;38:14-19.

Ultrasound-based quantification of submandibular space compliance as predictor for difficult intubation. A prospective observational preliminary study